JP2008037747A - Method for manufacturing silicon of solar grade - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an apparatus and a method for manufacturing silicon of a solar grade. <P>SOLUTION: The method for manufacturing silicon of the solar grade from a starting material containing a silica-containing granule and a carbon-containing film arranged in at least a part of the silica-containing granule comprises the steps of: heating the starting material to produce an intermediate; and further reacting the intermediate to produce the silicon of the solar grade. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates generally to a method for producing silicon, and more particularly to a method for producing solar grade silicon.

  Silicon is used in solar cells that convert solar energy into electrical energy. Silicon used in solar cells is of a quality called “solar grade” and its purity exceeds about 99.999%. In solar grade silicon it is very important that the content of group III and V electroactive elements of the Periodic Table of Elements, especially boron and phosphorus, is very low. This is because the presence of such impurities may adversely affect the performance of the solar cell.

The spread of solar cells and devices with solar cells is highly dependent on the availability and cost of solar grade silicon. In the photovoltaic industry, over 15,000 tons of silicon was used in 2005, and the amount is growing at a rate of about 30% every year. Several methods of producing solar grade silicon are known, most of which have one or more disadvantages in processing. Some of these methods are based on the carbothermal reduction of siliceous compounds such as silica, and high purity raw materials are required for the production of solar grade silicon.
US Pat. No. 4,247,528 US Pat. No. 4,439,410 British Patent No. 21081096 Yasuhiko SAKAGUCHI, Masato ISHIZAKI, Tetsuro KAWAHARA, Makoto FUKAI, Mitsugi YOSHIYAGAWA and Fukuo ARATANI .; “Production of High Purity Silicon by Carbothermic Reduction of Silica Using AC-arc Furnace with Heated Shaft”; ISIJ International. Vol, 32 (1992), No. 5, pp. 643-649

  Accordingly, it is desirable to provide a method that can address one or more of the problems discussed above in the production of solar grade silicon.

  In an embodiment of the present invention, a method for manufacturing silicon is provided. In this method, a starting material including a plurality of granules is prepared. The granules include silica and a film provided on at least a part of the granules, and the film includes carbon. The method further includes heating the starting material to produce an intermediate comprising silica and silicon carbide. The method further includes reacting the intermediate to produce silicon.

  These and other features, aspects and advantages of the present invention can be better understood with reference to the following detailed description taken in conjunction with the accompanying drawings. Throughout the drawings, like parts are given like reference numerals.

  In this description and in the claims, a number of terms are used and are defined to have the following meanings. Even in the singular form, it includes plural cases unless it is clear from the context. As used herein, the term “total pressure” refers to the sum of partial pressures of all components of a mixture. As used herein, the term “partial pressure” refers to the pressure of a particular component relative to the total pressure.

  The growth of the silicon-based solar energy industry is limited to some extent by the cost of solar grade silicon. The cost of solar grade silicon is attributed not only to the cost of processing but also to the raw material costs for manufacturing. Typical raw materials for silicon production include siliceous and carbon sources. In the method using a low cost raw material, more expensive processing conditions may be required to purify the obtained silicon. An object of the embodiment of the present invention is to solve such a problem.

  In an embodiment of the invention, solar grade silicon is produced by silica reduction using a hydrocarbon. Hydrocarbons are cheaper than other carbon sources such as high purity carbon black. Furthermore, liquid or gaseous hydrocarbons are highly pure and readily available. In yet another embodiment of the present invention, the use of granular raw materials increases the packing density of the raw materials in the reactor, which can further reduce processing costs.

  Referring to the drawings, FIG. 1 is a flowchart 10 of a solar grade silicon manufacturing method according to an embodiment of the present invention. In step 12, a starting material comprising a plurality of granules and a film deposited on at least a part of the granules is charged into the reactor. As used herein, the term “granule” refers to an individual unit of starting material, as opposed to a mass of solids, for example, a large block of raw material, and the term used herein is a micron-sized fine powder. Includes units from (for example, 325 mesh powder) to relatively large pellets of centimeter-sized material. The plurality of granules, in certain embodiments, have a median particle size of about 1 μm to about 150 μm. The plurality of granules may consist of pure silica or may be obtained by grinding large silica particles. In order to increase the purity of the silica, the granules are further refined with mineral acids such as, but not limited to, nitric acid, hydrochloric acid, hydrofluoric acid, aqua regia, hydrofluoric acid, sulfuric acid, perchloric acid, phosphoric acid and combinations thereof. You may wash. In other embodiments, the granules are agglomerates such as pellets. The median particle size of the pellets is typically in centimeters. In certain embodiments, the agglomerates are formed by mixing silica powder or particles with a binder to form a mixture, subjecting the mixture to drying, partial degradation or decomposition of the binder by evaporation of the solvent, baking or heating. The Examples of binders include hydrocarbons, saccharides, cellulose, carbohydrates, polyethylene glycols, polysiloxanes and polymeric materials. In some embodiments, the starting granule has a median particle size of about 150 μm to about 1 mm. In some embodiments, the starting granule has a median particle size of about 1 to about 5 mm. In other embodiments, the starting granule has a median particle size of about 5 mm to about 5 cm.

  The purity of the starting material greatly affects the properties of the final solar grade silicon. In certain embodiments, the purity of the starting material is about 99.999%. In another embodiment, the purity of the starting material is greater than about 99.9999%. Further, it is desirable that the amount of boron and phosphorus in the starting material is less than a predetermined limit. Typically, such an upper limit is about 50 ppb to about 500 ppb.

  Preparing the starting material 12 includes, in one embodiment, cracking hydrocarbons in at least a portion of the granule comprising silica. The decomposition of hydrocarbons is also called “hydrocarbon cracking”, and the hydrocarbons decompose to form a film made of carbon on the surface of the granules. An example of such a decomposition reaction is represented by the following chemical formula.

CH ₄ → C + 2H ₂
Here, methane decomposes as a hydrocarbon to produce carbon, which is deposited as a film on the granules. This is accompanied by the release of hydrogen gas. In one embodiment, the scavenging gas is passed through the zone where the above reaction occurs to remove the hydrogen gas. Specific examples of the scavenging gas include, but are not limited to, an inert gas such as argon, helium, or hydrogen.

  In certain embodiments, the coating is deposited on the surface of at least a portion of the granule. In certain embodiments, the coating is deposited on the surface of substantially all granules. In certain embodiments, the coating substantially covers the entire surface of the granules to be coated. When a film of carbon is deposited on a granule of silica, the surface area of carbon available for subsequent reactions with silica is significantly greater than the surface area available with conventional methods without such a film. This leads to an improvement in the yield of grade silicon. In an embodiment of the invention, the starting silica to carbon molar ratio is about 1: 2, according to the overall chemistry of the decomposition reaction.

  In certain embodiments, the cracking of the hydrocarbon includes heating the hydrocarbon to a temperature greater than about 600 degrees Celsius. In certain embodiments, the hydrocarbon decomposes at a temperature greater than about 1000 ° C. In some embodiments, the hydrocarbon consists of linear, branched or cyclic isomers of hydrocarbons, such as alkanes, alkenes, alkynes and aromatic hydrocarbon compounds. Specific examples of suitable hydrocarbons include, but are not limited to, natural gas, methane, butane, propane, acetylene, or combinations thereof. In certain embodiments, the hydrocarbon is a gas. In certain embodiments, the hydrocarbon is a liquid at ambient pressure and temperature.

  In step 14, the starting material is heated to produce an intermediate comprising silicon carbide and silica. The starting material is heated at a temperature above about 1600 ° C. In certain embodiments, the heating temperature is greater than about 1750 ° C. The intermediate formation reaction is represented by the following chemical formula.

3SiO ₂ + 6C → SiO ₂ + 2SiC + 4CO
Here, silica reacts with carbon to produce silicon carbide and silica, accompanied by the release of carbon monoxide gas. In certain embodiments, the silica consists essentially of liquid silica. In an embodiment of the present invention, the equilibrium of the reaction is maintained in the direction of intermediate formation (ie, to the right of the chemical formula) by keeping the partial pressure of carbon monoxide lower than that required for thermodynamic equilibrium. Can be shifted. In some embodiments, the partial pressure of carbon monoxide is less than about 50 kPa. In another embodiment, the partial pressure of carbon monoxide is less than about 25 kPa. In one embodiment, the partial pressure of carbon monoxide is maintained by flowing the scavenging gas described for the previous step through the intermediate. The flow rate of the scavenging gas depends on the design and configuration of the reactor for processing, the material packing density in the reactor, and the like. In one embodiment, the total reactor pressure is from about 100 to about 150 kPa. In another embodiment, the total reactor pressure is from about 150 kg to about 200 kPa. In yet another embodiment, the total reactor pressure is greater than about 200 kPa.

  In step 16, an intermediate comprising silicon carbide and silica is reacted to produce silicon. The intermediate is generally heated to a temperature above about 2000 ° C. In certain embodiments, the heating temperature is greater than about 2100 ° C. The solar grade silicon production reaction is represented by the following chemical formula:

SiO ₂ + 2SiC → 3Si + 2CO
Similar to the previous step, by keeping the carbon monoxide partial pressure below the level defined by the thermodynamic equilibrium, the reaction equilibrium can be shifted to the right side of the above formula, that is, the solar grade silicon production direction. In some embodiments, the carbon monoxide partial pressure is maintained at a pressure of less than about 90 kPa. In some embodiments, the partial pressure of carbon monoxide is maintained at a pressure less than about 46 kPa. In one embodiment, the carbon monoxide partial pressure is maintained by flowing a scavenging gas composed of an inert gas. In some embodiments, the resulting solar grade silicon consists essentially of liquid silicon.

  In some embodiments, an additional purification step is applied before using solar grade silicon. For example, it may be desirable to reduce the level of carbon or residual metal in silicon. Exemplary purification steps include removal by precipitation of particulates in molten silicon, removal of particulates in molten silicon by filtration, purification by blowing oxygen into the molten silicon, purification by blowing wet hydrogen into the molten silicon, Including zone purification, directional solidification or vacuum heating.

  In certain embodiments, the purity of solar grade silicon is greater than about 99.999%. In another embodiment, the purity of solar grade silicon is from about 99.999 to about 99.9999%. In some embodiments, the purity of solar grade silicon reflects the purity of the starting material.

  An example of a solar grade silicon manufacturing apparatus 20 according to an embodiment is shown in FIG. In one embodiment, the apparatus 20 is a reactor configured to receive a starting material comprising a plurality of granules from one end of the reactor and to recover a product comprising silicon at the opposite end of the reactor. . The apparatus 20 includes many components such as an inlet, an outlet, and a heat energy source. In some embodiments, the apparatus 20 comprises a multi-zone furnace and the temperature of each zone of the multi-zone can be controlled independently. Device 20 includes a housing 22 having a wall 24. In some embodiments, the housing 22 includes one or more walls 24. Wall 24 has an inner surface 26 that defines a chamber 28. The wall 24 of the housing 22 is made of metal, refractory material, graphite, silicon carbide or fused silica that meets the material requirements for solar grade silicon manufacturing. In the illustrated embodiment, the wall 24 of the device 20 is made of graphite. Further, an inert liner (not shown) may be provided on the inner surface 26 of the wall 24. The liner material is selected so that it is not a source of harmful contaminants. The liner can prevent or reduce material deposition on the inner surface 26 of the wall 24. Preferably, the liner may be removable so that deposits can be stripped from the wall 24 during the cleaning process.

  The configuration (for example, shape and size) of the wall 24 of the housing 22 can be set according to processing conditions. Its configuration depends on the size and number of parts. In one embodiment, the housing 22 is rectangular. In the illustrated embodiment, the device 20 is configured in a cylindrical, vertical configuration with an outer diameter of about 7.5 cm to about 5 m and a length of about 1 to about 10 m, but with a different size and / or shape. 20 can also be used. The chamber 28 has a predetermined volume. The chamber 28 of the apparatus 20 may be divided into a plurality of zones, a first zone 32, a second zone 34, and a third zone 36 based on the reaction in each zone, but with respect to the physical barrier between the zones. There are no conditions.

  A set of thermal energy sources 40, 42, and 44 may be disposed near the housing 22 to supply thermal energy to the chamber 28. In certain embodiments, the thermal energy sources 40, 42, 44 are independently controllable. In some embodiments, the first thermal energy source 40, the second thermal energy source 42, and the third thermal energy source 44 have different temperatures in the first zone 32, the second zone 34, and the third zone 36, respectively. Can be supplied. In one embodiment, the thermal energy is supplied with a heater. The heater can supply thermal energy by resistance heating or induction heating. Examples of the heater include, but are not limited to, a ceramic heater, a molybdenum heater, a divided furnace heater, a three-zone divided furnace heater, or an induction heater. When an induction heater is used, the wall 24 of the housing 22 can act as a susceptor for induction heating of the chamber 28. In some embodiments, a single thermal energy source may be provided near the housing so that different temperatures are supplied to each reaction zone.

  A set of inlets and outlets for allowing material to flow into and out of the chamber 28 is provided directly above the first zone 32 at the top of the device 20. A silica source inlet 46 and a hydrocarbon inlet 48 extend through the wall 24 and communicate with the chamber 28 for introducing the silica source and hydrocarbon, respectively. The silica source inlet 46 and the hydrocarbon inlet 48 may be made of graphite, silicon carbide or a refractory material.

  The silica source inlet 46 may include one or more serial gate valves (not shown) so that silica can be introduced into the chamber 28 without disturbing the atmosphere in the chamber 28. An exhaust unit, a replenishing unit, and a control unit for controlling the state in the chamber 28 may be provided upstream of the silica source introduction port 46.

  The end of the hydrocarbon inlet 48 may be located above the zone 32 as shown in FIG. In some embodiments, the end of the hydrocarbon inlet 48 may be located in the zone 32. Further, the hydrocarbon introduction port 48 may be provided with a built-in tube, and in order to lower the temperature in the hydrocarbon introduction port 48, a coolant is passed through the built-in tube (not shown) and the hydrocarbon introduction port 48. The decomposition of hydrocarbons in the inside may be suppressed. In one embodiment, the coolant includes an air stream such as argon or helium. In certain embodiments, the coolant comprises a liquid such as water, ethylene glycol or propylene glycol.

What is necessary is just to adjust the flow rate of the hydrocarbon to the chamber 28 so that a uniform carbon film may be formed in the whole starting material. Typical flow rates for gaseous hydrocarbons are from about 1000 to about 100,000 cm ³ / min, but for devices of different dimensions, the flow rate may vary depending on the silica feed rate. In some embodiments, an external pump may be provided to supply hydrocarbons to the chamber 28. The external pump has the advantage that the hydrocarbon flow rate can be further controlled.

  A gas outlet 50 provided at the top of the apparatus 20 extends through the wall 24 and communicates with the chamber 28. The inlets 46, 48 and the gas outlet 50 can be made of graphite, silicon carbide or refractory material. Moreover, in order to control the flow of the material to the chamber, valves (not shown) may be provided at the inlets 46 and 48 and the gas outlet 50 singly or together with the mass flow controller. In some cases, a dryer and / or purification means for further drying and / or purifying the starting material may be provided at the silica source inlet 46 and the hydrocarbon inlet 48.

  A silicon outlet 52 and a gas inlet 54 are provided at the lower end of the apparatus 20 below the third zone 36. The gas inlet 54 is an optional component that may be appropriately provided depending on the embodiment. The silicon outlet 52 and the gas inlet extend through the wall 24 and communicate with the chamber 28 through the third zone 36. In some embodiments, gas inlet 54 may include an external pump for flowing scavenging gas into chamber 28. Silicon outlet 52 and gas inlet 54 are made of refractory material suitable for the production of graphite, silicon carbide, silica or other solar grade silicon. In order to control the flow of material entering and exiting the chamber 28, the silicon outlet 52 and the gas inlet 54 may be provided with valves (not shown) alone or with a mass flow controller, opening and baffle. Chamber 28 may include one or more baffles, openings, frits, etc. to facilitate mixing. Further, the gas inlet 54 and the silicon outlet 52 may be provided with a dryer and / or a use point purification means for further drying and / or purifying the scavenging gas and solar grade silicon, respectively.

  The silica source inlet 46 introduces silica-containing granules into the chamber 28. The typical packing density of the granules in the chamber is from about 10 to about 50% by volume. The first thermal energy source 40 proximate to the first zone 32 is energized to raise the temperature in the chamber 28. The first thermal energy source 40 is energized to increase the temperature in the chamber 28, particularly the temperature of the first zone 32, to a predetermined temperature at a predetermined temperature increase rate. In certain embodiments, the predetermined temperature is greater than 600 degrees Celsius. In certain embodiments, the predetermined temperature is greater than 1000 degrees Celsius.

  The hydrocarbon is introduced into the chamber 28 from the hydrocarbon inlet 48. When the predetermined temperature is reached, particularly in the first zone 32, the hydrocarbons decompose at the surface of the granules to form a film containing carbon. In certain embodiments, as the granules flow from the first zone 32 to the second zone 34, the majority of the granules have a coating that includes carbon. The granules of the second zone 34 and the third zone 36 may also have a coating containing carbon. In certain embodiments, the carbon is in a substantially solid form.

  After the coating is formed, the second thermal energy source 42 is energized to supply thermal energy to the second zone 34 to raise the temperature of the granules. In certain embodiments, the temperature of the second zone 34 is greater than about 1600 ° C. In certain embodiments, the temperature of the second zone 34 is greater than about 1750 ° C. Granules containing silica and carbon form an intermediate containing silica and silicon carbide. Formation of the intermediate also releases silicon monoxide in the second zone 34 in certain embodiments. In one embodiment, a scavenging gas is flowed from the gas inlet 54 to keep the partial pressure of carbon monoxide below about 50 kPa. In another embodiment, a scavenging gas is flowed from the gas inlet 54 to keep the carbon monoxide partial pressure below about 25 kPa. The carbon monoxide generated in the chamber 28 is removed from the gas outlet 50 by the scavenging gas. The scavenging gas also carries away other unwanted gas species such as hydrogen and silicon monoxide. The flow rate of the scavenging gas may be adjusted so that the desired partial pressure of carbon monoxide is maintained. Furthermore, the flow rate of the scavenging gas is adjusted so that the vapor phase silicon monoxide produced in the second zone 34 can be condensed on the coated granules present in the first zone 32 and used for conversion to solar grade silicon. As appropriate, the gas outlet 50 may be monitored to detect the concentration of carbon monoxide discharged from the gas outlet 50, which can be an indicator of the reaction conditions in the chamber 28. In embodiments where the apparatus 20 does not include a gas inlet 54, other alternative techniques may be used without introducing the scavenging gas into the chamber 28 to control the reaction.

  Thermal energy is supplied by energizing the third thermal energy source 44 to raise the temperature of the third zone 36. In certain embodiments, the temperature of the third zone 36 is greater than about 2000 degrees Celsius. In certain embodiments, the temperature of the third zone 36 is greater than about 2100 ° C. The intermediate reacts to produce solar grade silicon and carbon monoxide is released. In some embodiments, the reaction of the intermediate also releases silicon monoxide in the third zone 36. In one embodiment, a scavenging gas is flowed from the gas inlet 54 to keep the carbon monoxide partial pressure below about 90 kPa. In another embodiment, a scavenging gas is flowed from the gas inlet 54 to keep the carbon monoxide partial pressure below about 46 kPa. The scavenging gas causes carbon monoxide to be exhausted from the chamber 28 through the gas outlet 50. Furthermore, the flow rate of the scavenging gas is adjusted so that the vapor phase silicon monoxide produced in the third zone 36 can be condensed on the coated granules present in the first zone 32 and used for conversion to solar grade silicon. Also good. An intermediate containing silica and silicon carbide reacts to produce silicon. In some embodiments, the silicon is liquid. Silicon may be collected at the lower end of the device 20 below the third zone 36 and discharged from the silicon outlet 52. This method may be a continuous method, in which material is continuously supplied from the upper part of the apparatus 20 and a product made of solar grade silicon is recovered from the silicon outlet 52 at the lower end of the apparatus 20.

  In one embodiment, a fourth zone (not shown) may be provided between the third zone 36 and the silicon outlet 52 to allow controlled cooling before the molten silicon is discharged from the chamber.

  In some embodiments, the reactor axis is tilted instead of a vertical arrangement. When the reactor is tilted at a predetermined angle from the vertical arrangement, the silica source inlet, the hydrocarbon inlet, and the gas outlet are provided above the gas inlet and the silicon outlet. In certain embodiments, the silica source inlet, hydrocarbon inlet, and gas outlet are provided at one end of the reactor, and the gas inlet and silicon outlet are provided at the opposite end of the reactor. In certain embodiments, the reactor may be rotated to facilitate mixing of the silica with the added hydrocarbon, or to facilitate transport of the coated granules below the reactor.

  While certain features of the invention have been illustrated and described herein, many modifications and changes will be apparent to those skilled in the art. Accordingly, the claims of the present application include all such modifications and changes belonging to the technical idea of the present invention.

The flowchart of the silicon manufacturing method which concerns on embodiment of this invention. The figure which shows the manufacturing apparatus of the silicon | silicone which concerns on embodiment of this invention.

Explanation of symbols

20 device 22 housing 24 wall 26 inner surface 28 chamber 32 first zone 34 second zone 36 third zone 40 first heat source 42 second heat source 44 third heat source 46 silica source inlet 48 hydrocarbon inlet 50 gas outlet 52 Silicon outlet 54 Gas inlet

Claims (10)

A silicon manufacturing apparatus (20),
A housing (22) comprising a wall (24) having an inner surface (26) defining a chamber (28);
A thermal energy source proximate to the housing (22) and capable of supplying thermal energy to the chamber (28);
A silica source inlet (46) at one end of the apparatus, the silica source inlet (46) communicating with the chamber (28) for introducing a silica source comprising a plurality of granules into the chamber (28); ,
A hydrocarbon inlet (48) at the end of the apparatus, the hydrocarbon inlet (48) communicating with the chamber (28) for introducing hydrocarbons into the chamber (28);
A gas outlet (50) at the end of the apparatus, the gas outlet (50) communicating from the chamber (28) to the outside of the chamber (28) and capable of exhausting gas from the chamber (28) to the outside of the chamber (28). 50),
A device comprising a silicon outlet at the opposite end of the chamber, the silicon outlet (52) communicating from the chamber (28) to the outside of the chamber (28) and capable of discharging generated solar grade silicon. In addition, a first thermal energy source (40) configured to increase the temperature of the first zone to greater than about 600 ° C. and a first thermal energy source (40) configured to increase the temperature of the second zone to greater than about 1600 ° C. The apparatus of claim 1, comprising two thermal energy sources (42) and a third thermal energy source (44) configured to raise the temperature of the third zone to greater than about 2000 degrees Celsius. The apparatus according to claim 1, wherein the apparatus comprises a multi-zone heating furnace, and the temperature of each zone of the multi-zone can be independently controlled. The apparatus of claim 1, further comprising a gas inlet (54) for allowing the sweep gas to flow into the opposite end of the apparatus. The apparatus of claim 1, wherein the hydrocarbon inlet (48) further comprises a built-in tube configured to allow coolant to flow through the tube. The apparatus of claim 1, wherein the hydrocarbon comprises an alkane, alkene, alkyne, aromatic hydrocarbon, or a combination thereof. The apparatus of claim 1, wherein the apparatus (20) is configured to decompose hydrocarbons to form a film comprising carbon on the plurality of granules, wherein the plurality of granules comprise silica. The apparatus of claim 1, wherein the median particle size of the plurality of granules is from about 1 μm to about 5 cm. The apparatus of claim 1, wherein the silica / carbon molar ratio in the plurality of granules is about 1: 2. The apparatus of claim 1, wherein the hydrocarbon and silica sources are continuously fed to one end of the apparatus (20) and solar grade silicon is recovered from the opposite end of the apparatus (20).

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